Remove panic!() in loop.cycle([])

askama_shared/src/generator.rs

@@ -1328,7 +1328,7 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
                 "cycle" => match args {
                     [arg] => {
                         if matches!(arg, Expr::Array(arr) if arr.is_empty()) {
-                            panic!("loop.cycle(…) cannot use an empty array.");
+                            return Err("loop.cycle(…) cannot use an empty array".into());
                         }
                         buf.write("({");
                         buf.write("let _cycle = &(");

testing/tests/ui/loop_cycle_empty.rs

@@ -0,0 +1,14 @@
+// Nb. this test fails because currently an empty array "[]" is always a syntax error in askama,
+// but even if this changes, this test should keep failing, but possibly with another error message
+
+use askama::Template;
+
+#[derive(Template)]
+#[template(
+    source = r#"{% for v in values %}{{ loop.cycle([]) }}{{ v }},{% endfor %}"#,
+    ext = "txt"
+)]
+struct ForCycleEmpty;
+
+fn main() {
+}

testing/tests/ui/loop_cycle_empty.stderr

@@ -0,0 +1,8 @@
+error: problems parsing template source at row 1, column 34 near:
+       "([]) }}{{ v }},{% endfor %}"
+ --> tests/ui/loop_cycle_empty.rs:6:10
+  |
+6 | #[derive(Template)]
+  |          ^^^^^^^^
+  |
+  = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)